1. Field of the Invention
The present invention relates to an optical disk and an optical disk apparatus, and, more particularly, to the configuration of synchronization information.
2. Related Art
An optical disk apparatus has hitherto optimized recording power by means of a technique called OPC (Optimum Power Control) or R-OPC (Running Optimum Power Control), thereby recording data. According to the OPC technique, test data are recorded in a test area (PCA) of the optical disk while the recording power is changed in various manners before recording of data, and the test data are reproduced. Recording power at which the quality (e.g., a β value) of the reproduced signal is optimized is selected as optimum recording power. In the meantime, according to the R-OPC technique, consideration is given to the fact that the sensitivity of a recording film of the optical disk is not necessarily uniform within the plane. When data are actually recorded at the optimum recording power determined through OPC, the quantity of return light arising at the time of recording of data is monitored. The recording power is controlled by feedback such that the quantity of return light assumes a constant value. In general, the quantity of light of level B is used as the quantity of return light. The level B represents the quantity of return light acquired as a result of formation of a pit when the optical disk is exposed to recording power (i.e., the quantity of return light arising as a result of the recording power having been diffracted by the pit). Specifically, when the quantity of return light of level B falls below a given value, this indicates that pits are formed to an excessive degree. Hence, the recording power is reduced. In contrast, when the quantity of return light of level B falls above the given value, this indicates that pits are formed to an insufficient degree, and the recording power is increased.
Data having lengths 3 T to 11 T (T is a reference clock length) are recorded on a CD-R/RW or the like. However, difficulty is encountered in monitoring the quantity of return light resulting from light being diffracted by pits having short lengths and controlling the recording power through feedback. In light of this difficulty, the quantity of return light arising at the time of recording of data having the longest pit length 11 T is detected, to thus effect R-OPC. The CD-R/RW complies with specifications of data of a length 11 T being included in the synchronization information (SYNC) and of two data sets, each having a length 11 T, successively appearing twice in each frame in all cases. Any one of the two data sets, each having a length 11 T, inevitably turns into a mark (i.e., a data set for which a pit is formed by radiation of recording power). Therefore, R-OPC is carried out periodically (i.e., at the timing of periodically-inserted synchronization information) through use of the data set having a length 11 T, thereby making an attempt to periodically optimize the recording power.
In the case of a DVD-R/RW or the like, pits are formed to lengths 3 T to 11 T as in the case of the CD-R or the like. However, a pit length 14 T is used for synchronization information. In contrast with the CD-R, the DVD-R/RW complies with specifications of data having a 14 T pit length not being successively inserted twice (i.e., a mark and a space are not inserted as a pair) but being inserted only once in one frame (93 bytes). Selection of the data set having a pit length 14 T as a mark or a space is arbitrary. For example, when all of data having a pit length 14 T are set as spaces, pits are not formed to the longest pit length 14 T, and hence R-OPC cannot be performed. Consequently, the recording power cannot be periodically optimized, which in turn poses difficulty in maintaining recording quality.
As a matter of course, R-OPC can also be considered to be enabled by means of selecting as marks all of the data having a 14 T pit length. However, when all of the data having a 14 T pit length are selected as marks, there may arise a fear of an increase in DSV, which results in an increase in low-frequency components. The DSV (Digital Sum Value) is a value determined by accumulating all bits in a sequence—which takes two statuses—from the top while one status (e.g., 1) of the bit sequence is taken as +1 and the other status (e.g., 0) of the same is taken as −1. The smaller the DSV, the smaller the low-frequency components (DC components), resulting in an improvement in recording and reproducing quality.
In view of the circumstances, Japanese Patent Laid-Open Publication No. 2003-91819 describes a technique for setting marks and spaces as synchronization information items such that they are arranged alternately. According to this technique, R-OPC, or the like, is periodically performed in accordance with the synchronization information about the marks, thereby enabling an attempt to optimize the recording power.
However, even when the synchronization information is arranged such that a mark and a space are arranged alternately, there arises a problem of the DSV not necessarily being suppressed. Specifically, the DSV is determined by synchronization information and modulated data subsequent thereto. When the polarity (i.e., being a mark or a space) of the synchronization information and the polarity of the modulated data are set such that the DSV is minimized; that is, when the synchronization information is uniformly set such that a mark and a space are arranged alternately, there arises a necessity for adjusting the DSV through use of only the modulated data, so that the DSV cannot be suppressed. Specifically, in the case of a DVD, the modulated data are of 8-16 modulation scheme. The DSV can be adjusted to a certain extent by selecting a 16-bit value which becomes an odd number for a data value 1 in accordance with an unmodulated 8-bit data, or a 16-bit value which becomes an even number for a data value 1. However, on some occasions such an adjustment is not sufficient. Particularly, when two modulated data are not prepared (i.e., a substitute conversion table is not prepared) for all of unmodulated data or when physical difficulty is encountered (e.g., two modulated data sets are prepared for 0 to 87 data sets among 0 to 255 unmodulated data sets, but only single modulated data are used for subsequent data), adjustment of the DSV becomes difficult.
Moreover, if the synchronization information is uniformly fixed such that a mark and a space are arranged alternately, areas which are to become marks arise at all times at the time of recording of data on, e.g., a rewritable optical disk, and a characteristic of the recording film will be deteriorated, thereby resulting in a fear of a decrease in the number of times data on the optical disk can be rewritten.